1. Field of the Invention
The present invention relates to a process for the preparation of polyester polyols and/or polyester-polyether polyols. More particularly, the invention relates to polyester polyols and/or polyester-polyether polyols prepared by the oxyalkylation of carboxylic acid half esters in the presence of a catalytic amount of at least one thiodialkylene glycol.
2. Description of the Prior Art
Halogen and ester group containing polyether polyols are frequently used as a component for the preparation of flame resistant polyurethane foams.
According to data in German Application No. 19 23 936 (U.S. Pat. No. 3,585,195), such polyester polyether polyols are obtained by the reaction of polyether polyols with halogen-containing carboxylic anhydrides, particularly tetrabromoor tetrachlorophthalic anhydride to form carboxylic acid half esters with subsequent oxyalkylation of the free carboxyl groups with alkylene oxide.
The oxyalkylation is commonly carried out in the presence of bases, for example, alkali hydroxides or alkali alcoholates or acids as catalysts. A drawback of this method is that the catalysts, due to their low selectivity, not only accelerate the oxyalkylation of the carboxyl groups but also accelerate the addition of alkylene oxides to the resultant or already present hydroxyl groups of the polyester or polyether polyols. In order to guarantee complete esterification of all carboxyl groups, the oxyalkylation must, therefore, be carried out with a large excess of alkylene oxides accompanied by a long reaction period. Another drawback is the fact that the catalyst must be separated from the reaction mixture at the completion of the reaction requiring cumbersome and expensive purification operations.
In accordance with German Pat. No. 2,724,609 the oxyalkylation of chlorine-containing phthalic acid half esters is preferably carried out in the absence of catalysts in order to avoid subsequent purification. The uncatalyzed oxyalkylation, however, also requires a large alkylene oxide excess in order to esterify all carboxyl groups. Another drawback is that the unreacted alkylene oxides, which represent up to 15 percent by weight of the amount used, must be removed by distillation and must be disposed of which effects the economy of this method as well as its environmental soundness.
Also well known is the esterification of carboxylic acids with alkylene oxides in the presence of catalysts such as sulfuric acid, sodium acetate, iron (III)-chloride and others (Methods of Organic Chemistry, vol. VIII, Houben-Weyl, Georg Thieme Publishers, Stuttgart 1952, pages 531-533).
For the preparation of polyurethanes of organic polyisocyanates and compounds with reactive hydrogen atoms, polyether polyols with highly reactive hydroxyl groups are needed for various areas of application. These may be produced by introducing primary hydroxyl groups, for example, by oxyethylation of commonly used polyether polyols. This process requires a large excess of ethylene oxide which results in hydrophilic polyoxyethylene segments which impart hydrophilic properties to the polyurethanes produced therefrom. The hydrophilic character, however, is a drawback for important areas of application of polyether polyols, for example, in the preparation of foams.
Hydrophilic polyurethane foams, for example, foams used for insulating purposes or as upholstery material result in an uncontrollable impairment of their mechanical properties by absorbing water, are increasingly subject to a hydrolytic decomposition, and the loss of thermal insulating capacity.